Enhanced targeting of stem-like solid tumor cells with radiation and natural killer cells

Natural killer (NK) cells are innate lymphocytes postulated to mediate resistance against primary haematopoietic but not solid tumor malignancies. Cancer stem cells (CSCs) are a small subset of malignant cells with stem-like properties which are resistant to chemo- and radiotherapies and are able to repopulate a tumor after cytoreductive treatments. We observed increased frequencies of stem-like tumor cells after irradiation, with increased expression of stress ligands on surviving stem-like cells. Ex vivo NK cells activated by low dose IL2 in vitro and IL15 in vivo displayed an increased ability to target solid tumor stem-like cells both in vitro and in vivo after irradiation. Mechanistically, both upregulation of stress-related ligands on the stem-like cells as well as debulking of non-stem populations contributed to these effects as determined by data from cell lines, primary tumor samples, and most relevant patient derived specimens. In addition, pretreatment of tumor-bearing mice with local radiation prior to NK transfer resulted in significantly longer survival indicating that radiation therapy in conjunction with NK cell adoptive immunotherapy targeting stem-like cancer cells may offer a promising novel radio-immunotherapy approach in the clinic.